Does Running Hard Hurt Your Fertility?

Running is one of the constants of my life. Pursued with more regularity than, at times, some basic hygiene practices, and with more passion than some relationships, running has been such a staple largely because it has a way of giving me exactly what I need. Sometimes that means peace and relaxation on an easy run through the woods, or a way to connect with friends while moving my body. Other times it’s the satisfaction of executing a hard workout and seeing my fitness improve. Working hard and seeing reward was particularly helpful when I was in graduate school, a time when my research efforts often seemed to be going nowhere, and it felt like I was banging my head against the wall for weeks or months on end. I would do workouts on the track late at night under the lights while I waited for experiments to finish, returning to lab drenched from the Texas heat and humidity. I ran my first half marathon during this time, then my first marathon, addicted to the cycle of training hard and seeing my race times fall.

A few years later, when I was trying to get pregnant, I was still in this cycle, which provided stability as I moved across the country and started a new job. Intuitively it felt like I should scale back on the intensity of my efforts. Easy runs where I could talk comfortably seemed probably okay, beneficial even, but efforts involving running as hard as I could on heavy legs, feeling like I was on the verge of (or actually) throwing up did not seem conducive to creating or supporting life. The information I got from providers was that any type of exercise was fine. But this just didn’t feel right to me.

My instincts, it turns out, were onto something. Two large studies directly link the intensity and duration of exercise to lower fertility and cycle disruption. One, conducted in Norway with about 4,000 women trying to conceive, found that frequency of exercise alone correlated with problems conceiving — women who were active most days were about three times more likely to have fertility problems than inactive women.¹ Results were even worse for women who exercised vigorously, defined as working out to the point of exhaustion. These women were over two times more likely to have fertility problems than the low intensity exercisers. A second study of several thousand Danish women trying to conceive found similar results. Women who worked out vigorously for two hours a week were about one fifth less likely to be pregnant than inactive women over the same time frame.² This percentage increased, to about a quarter less likely, for women exercising 3-4 hours a week at this intensity, and to about a third less likely at five or more hours per week.

These results made intuitive sense to me. But what was the mechanism behind this fertility dip?

Researchers have identified a threshold below which the body starts making difficult decisions: when energy availability drops below roughly 30 calories per kilogram of lean body mass per day, the signaling that drives ovulation begins to break down.³ The body, reading scarcity, starts deprioritizing reproduction. This prioritization is driven not by exercise but by energy deficit: the same disruption can be induced by under-eating with or without training.⁴⁵ But this number is not a binary cut-off — menstrual disturbances can happen to varying degrees above and below that threshold that can be best understood not as a switch but as a rough indicator of hormonal trouble.⁵

The clearest confirmation of this came from a controlled study that found women who exercised vigorously but ate enough to compensate showed no menstrual disturbances, while those running a deficit, whether through exercise, undereating, or both, did.⁶ This also explains a detail from one of the large cohort studies that had initially puzzled me: women with higher BMIs weren’t affected by vigorous exercise the way leaner women were.² More body fat means more energy reserves, which means the body is less likely to read exercise as scarcity.

Historically I had not been particularly good about replenishing calorically after hard workouts. Part of the problem was that I often would just not be very hungry. Even after running 15 miles at a hard pace, I would not feel compelled to eat more than toast. This is not just a me thing. Called exercise-induced anorexia, feelings of hunger are often suppressed with vigorous aerobic exercise, and this suppression scales with intensity.⁷ One small study found that calorie intake was higher after moderate intensity efforts than high intensity efforts.⁸

RED-S is a well-known phenomenon where low energy availability causes cessation of menstruation and therefore fertility.⁹ But it seems that even lower levels of energy unavailability can disrupt the menstrual cycle as to affect fertility. My intuition about vigorous exercise and fertility was not completely wrong, but slightly misguided. An intense, hard effort run can be challenging to replenish from calorically, and refueling should be deliberate, not based on hunger signals. I didn’t need to be worried about hard track workouts where I pushed to my physical and mental limits. I needed to focus on refueling with hearty meals instead of snacking on toast.